Catheterization procedures have become the preferred methods for treating a variety of heart conditions. A catheter has a long, flexible body that may be introduced into the vasculature and guided into the heart or another organ of the body. By incorporating various medical devices, for example an ablation device, into the distal portion of a catheter, it may be used for ablation, as well as other procedures including angioplasty, dilation, biopsy and other procedures within the human heart and elsewhere.
For certain cardiovascular procedures, a catheter is inserted into an artery or vein in the leg, neck, torso or arm and threaded through the vasculature into the heart. FIG. 1 (prior art) shows a human heart 10 into which a catheter 12 has been introduced. In a common procedure, a catheter 12 enters into the right atrium 14 through the inferior vena cava 16. From the right atrium 14, the catheter may be directed to other regions of the heart.
Many procedures require introducing the catheter to the left atrium 18. One method of entering the left atrium 18, known as a trans-septal procedure, includes first entering the right atrium 14 through the vena cava 16 and then puncturing the interatrial septum 20. The interatrial septum 20 is the portion of the atrial wall which divides the left and right atria. Once the interatrial septum 20 is punctured, the end of the catheter 12 proceeds into the left atrium 18 where it may perform ablation or other medical treatments.
At the center of the interatrial septum 20 is a thin fibrous region known as the fossa ovalis 22. The fossa ovalis 22 is surrounded by the muscular tissue 24 of the atrial walls. The fossa ovalis 22 is relatively thin and does not include muscular tissue, making it the generally preferred region in which to puncture the interatrial septum during trans-septal procedures. Puncturing of the muscular tissue surrounding the fossa ovalis causes unwanted damage to the heart.
Accurate localization and identification of the fossa ovalis presents significant challenges. The fossa ovalis is invisible to current imaging techniques used to visualize a catheter during intercardial procedures, such as fluoroscopy. As a result, a physician must rely upon his own skill to position the tip of a catheter prior to puncturing the fossa ovalis.
Another difficulty encountered during trans-septal procedures is the variance in the thickness of the fossa ovalis between different individuals. Without knowing the approximate thickness of the fossa ovalis being punctured, a surgeon does not know how far a puncturing device on a catheter must travel or how much force is required in order to traverse the fossa ovalis. Thus an additional risk inherent in trans-septal procedures is that too much force will be applied to the catheter, resulting in piercing both the fossa ovalis and other structures within the heart such as the heart wall or aorta in unnecessary and undesirable locations.
One method of identifying the fossa ovalis uses spectroscopy and the ability of light to penetrate some thickness of tissue. The blood in the right atrium, where the catheter is introduced into the heart, is filled with deoxygenated blood that has a characteristic absorption spectrum 30 shown in FIG. 2. The absorption spectrum 30 of deoxygenated, right atrium blood differs significantly from the absorption spectrum 32 of the oxygenated blood found in the left atrium, particularly within the range of 600 nm to 805 nm. Because the fossa ovalis is very thin, the oxygenated blood of the left atrium may be viewed by an optical device abutting the fossa ovalis.
The fossa ovalis is the only region of the right atrium where oxygenated blood and its characteristic spectrum may be observed. Thus, a catheter may incorporate an optical device that illuminates the blood and tissue surrounding the catheter tip and detect the spectrum reflected back. When such an optical device detects the absorption spectrum of oxygenated blood, the device is abutting the fossa ovalis. Thus, by moving a catheter tip about the right atrium and particularly around the atrial wall of the right atrium, an operator of a catheter device may identify the fossa ovalis prior to puncturing the interatrial septum.
However, the nature of the optical device may create difficulties in accurately identifying the fossa ovalis. The light being reflected and observed by the detector must travel far enough to be reflected by the oxygenated blood of the left atrium. Where an optical device has an emitter for illuminating surrounding tissue and a detector for observing the absorption spectra positioned very close to each other, most of the observed reflected light is reflected from the fossa ovalis itself and not the oxygenated blood of the left atrium. Thus, the fossa ovalis may not be accurately localized.
If, on the other hand, the emitter and detector are relatively far apart, the detector may observe light reflected from the left atrium even though it traveled through the muscular wall, not the fossa ovalis. As a result, a portion of the interatrial septum having muscular tissue may be mistakenly identified as the fossa ovalis. Thus, a trans-septal puncture may be inadvertently made through muscular tissue, causing unnecessary damage.
It is therefore desirable to provide a system and method for identifying the location of a medical device within a patient and particularly for accurately identifying when a medical device is abutting the fossa ovalis. It is also desirable to be able to measure light reflected from different distances from the tip of the catheter within the body.